During their production, single-walled carbon nanotubes form bundles. Owing to the weak van der Waals interaction that holds them together in the bundle, the tubes can easily slide on each other, resulting in a shear modulus comparable to that of graphite. This low shear modulus is also a major obstacle in the fabrication of macroscopic fibres composed of carbon nanotubes. Here, we have introduced stable links between neighbouring carbon nanotubes within bundles, using moderate electron-beam irradiation inside a transmission electron microscope. Concurrent measurements of the mechanical properties using an atomic force microscope show a 30-fold increase of the bending modulus, due to the formation of stable crosslinks that effectively eliminate sliding between the nanotubes. Crosslinks were modelled using first-principles calculations, showing that interstitial carbon atoms formed during irradiation in addition to carboxyl groups, can independently lead to bridge formation between neighbouring nanotubes.
Novel magnesium matrix composites reinforced with carbon nanotubes have been processed by powder metallurgy. Blends of metal powders and multi-wall carbon nanotubes were compacted by uniaxial hot pressing followed by hot isostatic pressing. A uniform dispersion of nanotubes in the metal matrix was obtained. A coating method of nanotubes is described, which is promising to improve the matrixreinforcement bonding strength. R54 E. Carreño-Morelli et al.: Carbon nanotube/magnesium composites
We carried out in situ resistivity measurements on macroscopic oriented ropes of single wall carbon nanotubes in a transmission electron microscope. We have found a minimum in the resistivity as a function of irradiation dose. This minimum is interpreted as a result of a twofold effect of the irradiation: the domination of covalent bond formation between tubes in a bundle due to broken bonds in the tube walls and the amorphization of the sample at high dose. Despite this improvement, the temperature dependence of the resistivity remains hopping-like.
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